CONCRETE AGGREGATES While cement, in concrete construction, is a very important element, nevertheless the other materials with which it is combined, and the manner of proportioning, mixing, and depositing them, are also factors of prime importance, and should be submitted to the same carefulness of inspection, preliminary testing, and intelligent supervision of competent authority as may be required of the cement itself.
The solid materials which are mixed with cement and water to form mortar and concrete, are known under the common name of Aggre gates. The substances chiefly used as aggregates are sand, crushed stone, and gravel. The stone may range from hard trap rock and granites to the weaker and more porous varieties of sand stone, etc.; and the list of available materials includes also shells, slate, shale, cinders, slag, crushed lava, broken brick, metal filings, and— for "pulp" concrete—sawdust.
A fairly good concrete can be made with almost any mineral aggregate, provided enough cement is used. In order, however, to save cement, reduce cost, and at the same time secure the best results, it is exceedingly important that the concrete worker know how to select the best aggregates for the work in hand, and how to determine the proper proportions to use of fine and coarse ingredients.
Selecting Aggregates In the first place, an aggregate for any kind of work should be clean. Dirt is a sign of weak ness. Any foreign substance, such as mud or clay, forming a coating on the particles of the aggregate, prevents the cement from coming into contact with the surface, and lowers the cohesive strength of the concrete, or its power to hold itself together. It may also seriously retard the setting. A loss of 15 to 20 per cent in strength may be caused by the use of a dirty aggregate.
Coarse aggregates can be cleaned by spread ing them on an inclined plane, turning a hose on the pile, and allowing the water to carry away the impurities. Fine aggregates can also be cleaned in a similar way, by spreading on an inclined platform in a layer 3 or 4 inches deep, and washing gently with a hose from the high end of the platform. Pieces of board pro
jecting up about 4 inches above the face of the platform, should be nailed around its edges to confine the sand, the water being allowed to flow over the top of the board nailed across the low end. • A small quantity of clay or loam will not injure the sand, but any amount over 5 per cent should be washed out.
On a very small job, the sand may be cleaned by stirring up in water in pails or barrels, and pouring or drawing off the water while the dirt is still in suspension.
It may sometimes be advisable to use the aggregate that is nearest at hand, and to depend for results on an extra amount of cement. A natural bank of sand. and gravel, for example, may be located near the work, giving an abun dant supply of aggregate at a cost of very little more than the labor required to get it out. If the structure is to be a small one or of compara tively slight importance—say, a culvert—and particularly if the cost of Portland cement is low, the above course is the proper one to follow. But in the case of large or important structures such as buildings and bridges, or where special products such as concrete blocks or brick are being manufactured, the aggregates and cement should be selected and proportioned with the greatest care, to insure uniformity and reliabil ity of results.
In the selection of aggregates, it is well to bear in mind that the object is to make a mixture in which all the voids or spaces between the particles shall be filled. The voids in the stone or gravel should be filled by the sand, and the voids in the sand should be filled by the cement paste. This result is secured by varying the pro portions of coarse and fine material. Since a perfect filling of the voids is rarely if ever attained, it is well to use a little more sand and a little more cement than would be just enough to fill them.